Checking transmission fluid level with tool

The "proper" way is to use that tool a scan tool to read the fluid temperature and consult a chart to tell you what the level is. The way to do it if you don't have a scan tool is to let the vehicle set so the fluid is at ambient temperature above 60F (15c). Start the engine, run the trans through the gears quick, put it in park and slide the tool in until it hits the bottom of the pan. Pull it out and read the number. At sixty you want to see 20mm. or a bit above.

A way to check it hot without a scan tool is to get a non contact IR thermometer. Now follow the above procedure but run the truck until it is fully warmed up. Use the thermometer to get the fluid temperature by reading the temperature of the transmission pan. Add about ten degrees F as the pan will not be at full temperature. Then do the same thing.
Those give you a "normal cold" and a "normal hot" number. The book allows for 5mm on either side as "normal"

Now if you have a trans temperature gauge, just use that temperature and the chart.